Ar15 Bolt won't lock back on last round

[Jwmeador]

My father just called me to tell me his AR 15 will lock back every time with 30 round magazines, but with 20 round magazines (4 different brands) the bolt won't lock back at all. Any ideas? Thanks in advance.

 

[Squirt239]

Magazine follower or mag spring.

 

[throttletony]

My father just called me to tell me his AR 15 will lock back every time with 30 round magazines, but with 20 round magazines (4 different brands) the bolt won't lock back at all. Any ideas? Thanks in advance.

That's odd.
I was going to say it's undergassed, but if it does it with 30 rounders, that's odd.
Keep us posted


Is it an old gun? Has it been used a lot?
Did he build it? Did he check headspace and does it go into battery well?
Can he try a different buffer/spring combo?

 

[Jwmeador]

That's odd.
I was going to say it's undergassed, but if it does it with 30 rounders, that's odd.
Keep us posted


Is it an old gun? Has it been used a lot?
Did he build it? Did he check headspace and does it go into battery well?
Can he try a different buffer/spring combo?

No it is not an old gun. Yes he built it himself. And he has not checked headspace yet. Just out of curiosity how would a headspace issue cause the issue?

 

[Bfish]

Is this with multiple 30 rounders or just one??

 

[17 squirrel]

How old are the 20 round Mags ????
Are they vintage AR Mags from the 60's or 70's ???

 

[Bennettjh]

Magazine follower or mag spring.

My guess as well.

 

[Fargo]

No it is not an old gun. Yes he built it himself. And he has not checked headspace yet. Just out of curiosity how would a headspace issue cause the issue?

Don't know why headspace would affect it, it sounds to me like 17 squirrel is on the right track. Old 20 rounders usually don't have the same spring strength as new 30's.

 

[NyleRN]

How old are the 20 round Mags ????
Are they vintage AR Mags from the 60's or 70's ???

This is the question that needs to be answered

 

[Jwmeador]

This is the question that needs to be answered

The 20 round magazines were anywhere from 5-8 years old but used very lightly. The round mags were about the same age

 

[17 squirrel]

Are these GI Mags or aftermarket ?? Colt ?
I would be tempted to stretch a Spring in one a inch or two and see how that works. If it works there's you answer....

 

[Zero_ek]

I had an old Colt 20 rounder that wouldn't lock back on the last round. I think it was an old school one and the follower looked different than any newer mag.

 

[Fargo]

I had an old Colt 20 rounder that wouldn't lock back on the last round. I think it was an old school one and the follower looked different than any newer mag.

The old original 20s had a metal follower if memory serves. I'm curious what kind these are.

 

[Leo]

SInce it appears the same ammo was used in both mags, it is less likely to be underpowered ammo. I have seen some bolt releases that are just a little undersize, double check that an empty magazine is actually catching on the tab at the back. Take the springs out of the magazines, stretch them until they are about 6 inches longer, put them back together. I cannot begin to count how many times this has worked since the 1980's. If it does not work, at least it didn't cost anything. Good luck.

 

[Leo]

The old original 20s had a metal follower if memory serves. I'm curious what kind these are.

You're correct. I never had any problem with the aluminum ones. I have some Vietnam era magazines that have been in my possession for 30 plus years and they have been 100% reliable.

 

[1911ly]

It would be fairly easy to see if it is a gas problem. Pull the bolt all the way back on a empty mag as see if it catches. If it doesn't, then remove the upper. Put a mag in the lower. Push on the bolt release and see if the mag follower drops a little bit. If it doesn't then look at the mag follower,spring tension or the paw on the bolt release that rides on top the mag follower. There is a little tab that rest on the top of the follower. That pushes the follower down enough to let the bolt pass. It's either under gas, the paw or the mag follower or the follower spring not pushing the follower up high enough.

Using the 2 mags it will be easy enough to tell what is going on with the upper off. If you need pics or anything just yell. I or someone else here can take pics of the tab.

Oh, Does the bolt release push soft or firm. The spring for that could be an issue. It takes a firm push. If it is soft or sticky it will let the the bolt drop. It's not to hard to trouble shoot. Just yell. We'll get you threw it.

I am going to agree with others, It's mag follower or mag spring.

 

[Jwmeador]

Thanks for all your responses. I just figured out it was actually the aftermarket strike industries mag catch. I was designed different than mil spec and when i replaced with standard milspec the bolt locked back 100 percent of the time reguardless of magazine used. Thanks Again

 

[AmmoManAaron]

Thanks for all your responses. I just figured out it was actually the aftermarket strike industries mag catch. I was designed different than mil spec and when i replaced with standard milspec the bolt locked back 100 percent of the time reguardless of magazine used. Thanks Again

Glad you got it figured out. I've found that about 99% of the time milspec is the best spec!

 

[seedubs1]

My bad, just read upstream that the OP already figured it out. Regardless, my method of narrowing this down would have been as follows:

The question really is.....Does the bolt lock back when manually cycling the action with 30 round mags inserted?

If it locks back while manually cycling the action, then the bolt isn't moving rearward enough and you need to re-evaluate your gas/buffer system.

If it does not lock back while manually cycling the action, then look at the bolt catch, follower, bolt, and associated parts to figure out why it's not locking back with the 30 rounders.

 

[trophyhunter]

You're correct. I never had any problem with the aluminum ones. I have some Vietnam era magazines that have been in my possession for 30 plus years and they have been 100% reliable.

I still run my original 60's era Colt 20 round mags, same spring and follower since day one having seen 10's of thousands of rounds at Perry over the years and they're still 100% reliable.

OP, have dad press the mag release button all the way in and take a turn or three off the mag catch on the other side that should alleviate it if it's not under gassed or over buffered.

On Edit: Missed OP had sorted it before posting